OLED display device and pixel driving circuit thereof

ABSTRACT

A pixel driving circuit using 4T1C pixel structure applied in an OLED display device is provided. The OLED display device senses a threshold voltage imposed on the TFT when the OLED display device is powered off or powered on and compensates the threshold voltage which is sensed in normal operating display for the driving current of the OLED, thereby reducing the influence of the threshold voltage imposed on the TFT on the driving current of the OLED and improving the display quality of the OLED display device.

BACKGROUND 1. Field of the Disclosure

The present disclosure relates to the field of display technology, andmore particularly, to a pixel driving circuit for an organiclight-emitting diode (OLED) display device and the OLED display devicewith the pixel driving circuit.

2. Description of the Related Art

Recently, an organic light-emitting diode (OLED) display device has beena very popular and new flat display product worldwide because the OLEDdisplay device has features of auto-luminescence, wide viewing angles,short response time, high luminous efficacy, wide color gamut, lowoperating voltage, small thickness, potential to produce a displaydevice with large sizes and flexibility, and simple manufacturingprocess. Besides, the OLED display device costs less to a larger extent.

The TFT with a capacitor storage signal controls the brightness andgrayscale of the OLED in the OLED display device. To achieve the goal ofthe constant current driving, each of the pixels needs to be formed bytwo or more TFTs and a storage capacitor, that is, a 2T1C mode. FIG. 1is a circuit diagram of a pixel driving circuit arranged in an OLEDdisplay device of related art. As FIG. 1 illustrates, the pixel drivingcircuit of the OLED display device of related art includes two TFTs anda capacitor. Specifically, the pixel driving circuit includes aswitching TFT T1, a driving TFT T2, and a storing capacitor Cst. Thedriving current flowing through the OLED is controlled by the drivingTFT T2. The current measures I_(OLED)=k(V_(gs)−V_(th))² where kindicates an intrinsic conducting factor of the driving TFT T2 anddetermined by the characteristics of the driving TFT T2; V_(th)indicates a threshold voltage of the driving TFT T2; V_(th) indicatesthe voltage imposed on a gate and a source of the driving TFT T2. Thethreshold voltage Vth of the driving TFT T2 drifts in the long-timeoperation so the driving current flowing through the OLED changes,thereby causing poor display of the OLED display device and affectingthe quality of display images.

SUMMARY

To solve the technology of the related art, an object of the presentdisclosure is to propose a pixel driving circuit and an organiclight-emitting diode (OLED) display device with the pixel drivingcircuit. The pixel driving circuit is arranged in the OLED displaydevice and can diminish the threshold voltage imposed on a thin-filmtransistor (TFT) which affects the driving current flowing through theOLED.

According to a first aspect of the present disclosure, a pixel drivingcircuit for an organic light-emitting diode (OLED) display deviceincludes a first thin-film transistor (TFT), a second TFT, a third TFT,a fourth TFT, a capacitor, and an OLED. The first TFT includes a gateelectrically connected to a first node, a source electrically connectedto a second node, and a drain electrically connected to a first supplyvoltage. The second TFT includes a gate receiving a second scanningsignal and a drain electrically connected to the first node. The thirdTFT includes a gate receiving a first scanning signal and a drainelectrically connected to the first node. The fourth TFT includes a gatereceiving a third scanning signal and a drain electrically connected tothe second node. The capacitor is electrically connected between thefirst node and the second node. The OLED includes an anode electricallyconnected to the second node and a cathode electrically connected to asecond supply voltage. When the OLED display device is powered off orpowered on, a source of the second TFT receives the first data signal. Asource of the third TFT and a source of the fourth TFT receive aninitialized signal or a threshold voltage sensor. The threshold voltagesensor is configured to sense a threshold voltage of the first TFT andgenerate a threshold voltage signal. When the OLED display deviceoperates normally, the source of the second TFT receives a second datasignal formed by a combination of the threshold voltage signal and a rawdata signal. The source of the third TFT and the source of the fourthTFT receive the initialized signal. The initialized signal and the firstdata signal are both at constantly low voltage level. The raw datasignal is at single-pulse high voltage level.

Furthermore, the pixel driving circuit performs a reset operation and athreshold voltage sensing operation when the OLED display device ispowered off or powered on.

Furthermore, when the pixel circuit performs the reset operation, thefirst scanning signal is at low voltage level, the second scanningsignal and the third scanning signal are both at high voltage level, thefirst data signal is at low voltage level, and the source of the thirdTFT and the source of the fourth TFT receive the initialized signal.

Furthermore, when the pixel circuit performs the threshold voltagesensing operation, the first scanning signal is at low voltage level,the second scanning signal and the third scanning signal are both athigh voltage level, the first data signal is at low voltage level, andthe source of the third TFT and the source of the fourth receive thethreshold voltage sensor.

Furthermore, the pixel driving circuit performs the reset operation, athreshold voltage sensing operation, a threshold voltage compensatingoperation, and a driving operation when the OLED display device is innormal display.

Furthermore, when the pixel circuit performs the reset operation, thefirst scanning signal and the third scanning signal are at high voltagelevel, the second scanning signal is at low voltage level, and thesecond data signal is a sum of the reference signal at low voltage leveland the threshold voltage signal.

Furthermore, when the pixel circuit performs the threshold voltagesensing operation, the first scanning signal and the third scanningsignal are at low voltage level, the second scanning signal is at highvoltage level, and the second data signal is a sum of the referencesignal at low voltage level and the threshold voltage signal .

Furthermore, when the pixel circuit performs the threshold voltagecompensating operation, the first scanning signal and the third scanningsignal are both are low voltage level, the second scanning signal is athigh voltage level, and the second data signal is a sum of the displaydata signal at high voltage level and the threshold voltage signal.

Furthermore, when the pixel driving circuit performs the drivingoperation, the first scanning signal and third scanning voltage are atlow voltage level, the second scanning signal at low voltage level, andthe second data signal is the sum of the reference signal at low voltagelevel and the threshold voltage signal.

In a second aspect of the present disclosure, an organic light-emittingdiode (OLED) display device comprising the pixel driving circuit asprovided above.

The present disclosure has beneficiary effects as follows. The OLEDdisplay device senses a threshold voltage imposed on the TFT when theOLED display device is powered off or powered on and compensates thethreshold voltage which is sensed in normal operating display for thedriving current of the OLED, thereby reducing the influence of thethreshold voltage imposed on the TFT on the driving current of the OLEDand improving the display quality of the OLED display device.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described below in detail with reference to theaccompanying drawings, wherein like reference numerals are used toidentify like elements illustrated in one or more of the figuresthereof, and in which exemplary embodiments of the invention are shown.

FIG. 1 is a circuit diagram of a pixel driving circuit arranged in anOLED display device of related art.

FIG. 2 illustrates a schematic diagram of an organic light-emittingdiode (OLED) display device according to a first embodiment of thepresent disclosure.

FIG. 3 illustrates an equivalence circuit diagram of the pixel structureof the OLED display device according to the first embodiment of thepresent disclosure.

FIG. 4 is a timing diagram of each of the operating stages of the pixeldriving circuit when being turned off or on according to the embodimentof the present disclosure.

FIGS. 5A and FIG. 5B are operating flowcharts of the pixel drivingcircuit when being turned off or on according to the embodiment of thepresent disclosure.

FIG. 6 is a timing diagram of each of the operating stages of the pixeldriving circuit in normal display according to the embodiment of thepresent disclosure.

FIGS. 7A to FIG. 7D are operating flowcharts of the pixel drivingcircuit in normal display according to the embodiment of the presentdisclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present application are illustrated in detail in theaccompanying drawings, in which like or similar reference numerals referto like or similar elements or elements having the same or similarfunctions throughout the specification. The embodiments described belowwith reference to the accompanying drawings are exemplary and areintended to be illustrative of the present application, and are not tobe construed as limiting the scope of the present application.

In the drawings, thickness of layers and areas is exaggerated forclarify. In addition, the same element illustrated in drawings islabeled as the same number.

Please refer to FIG. 2 illustrating a schematic diagram of an organiclight-emitting diode (OLED) display device according to a firstembodiment of the present disclosure.

Please refer to FIG. 2. The OLED display device includes a display panel100, a scanning driver 200, and a data driver 300. The OLED displaydevice further includes other proper devices such as a timing controllerwhich controls the scanning driver 200 and the data driver 300, a powervoltage generator which supplies power positive electrode voltage andpower negative electrode voltage, etc.

Specifically, the display panel 100 includes a plurality of pixels PXarranged in an array, N scanning lines G₁ to G_(N), and M data lines D₁to D_(M). The scanning driver 200 is connected to the scanning lines G₁to G_(N) and drives the scanning lines G₁ to G_(N). The data driver 300connected to the data lines D₁ to D_(M) drives the data lines D₁ toD_(M).

Each of the plurality of pixels PX may be supplied with a scanningsignal or a plurality of scanning signals by the scanning driver 200.Each of the plurality of pixels PX may be supplied with a data signal bythe data driver 300. Both of them will be detailed later.

Each of the pixels PX includes a pixel driving circuit. The pixeldriving circuit proposed by the present disclosure is detailed asfollows.

Please refer to FIG. 3 illustrating an equivalence circuit diagram ofthe pixel structure of the OLED display device according to the firstembodiment of the present disclosure.

Please refer to FIG. 3. The structure of each of the pixels PX is a 4T1Cpixel according to the present embodiment of the present disclosure. The4T1C pixel includes an organic light-emitting diode (OLED), a firstthin-film transistor (TFT) T1, a second TFT T2, a third TFT T3, a fourthTFT T4, and a capacitor C.

A gate of the first TFT T1 is electrically connected to a first node a.A source of the first TFT T1 is electrically connected to a second nodeb. A drain of the first TFT T1 is electrically connected to a firstsupply voltage Vdd.

A gate of the second TFT T2 receives a second scanning signal Scan2. Adrain of the second TFT T2 is electrically connected to the first nodea.

A gate of the third TFT T3 receives a first scanning signal Scan1. Adrain of the third TFT T3 is electrically connected to the first node a.

A gate of the fourth TFT T4 receives a third scanning signal Scan3. Adrain of the fourth TFT T4 is electrically connected to the second nodeb.

A terminal of the capacitor C is electrically connected to the firstnode a, and another terminal of the capacitor C is electricallyconnected to the second node b.

An anode of the OLED is electrically connected to the second node b. Acathode of the OLED is electrically connected to a second supply voltageVss.

The first TFT T1 is a driving TFT.

When the OLED display device is powered off or powered on (or thepredetermined time after the OLED display device is powered off orpowered on), a source of the second TFT T2 receives a first data signalDATA1, and a source of the third TFT T3 and a source of the fourth TFTT4 receive an initialized signal INI or a threshold voltage sensor 400.The threshold voltage sensor 400 is configured to sense the thresholdvoltage Vth of the first TFT T1 and generate a threshold voltage signal.The operating process of the threshold voltage sensor 400 is detailed inthe following.

When the OLED display device operates normally (i.e., from the time whenthe display device is turned on or the predetermined time after thedisplay device is turned on to the time when the display device isturned off), the source of the second TFT T2 receives the second datasignal DATA2 formed by a combination of the threshold voltage signal andthe raw data signal. The source of the third TFT T3 and the source ofthe fourth TFT T4 receive the initialized signal INI.

In the reset stage, as FIG. 4 and FIG. 5A illustrate, the first scanningsignal Scan1 is at low voltage level. The second scanning signal Scan2and the third scanning signal Scan3 are both at high voltage level. Thefirst data signal DATA1 is at low voltage level VA. The source of thethird TFT T3 and the source of the fourth TFT T4 receive the initializedsignal INI. The initialized signal INI is at low voltage level Vini. Atthis time, the third TFT T3 is turned off. The second TFT T2 and thefourth TFT T4 are both turned on. The voltage imposed on the first nodea is Va=VA and the voltage imposed on the second b node is Vb=Vini,resulting in Vini=VA, which completes initialization.

At the threshold voltage sensing stage, as FIG. 4 and FIG. 5Billustrate, the first scanning signal Scan1 is at low voltage level; thesecond scanning signal Scan2 and the third scanning signal Scan3 areboth at high voltage level; the first data signal DATA1 is at lowvoltage level VA; the source of the third TFT T3 and the source of thefourth TFT T4 receives the threshold voltage sensor 400. At this time,the third TFT T3 is turned off, and the second TFT T2 and the fourth TFTT4 are both turned on. The voltage imposed on the first node a is Va=VAand the voltage imposed on the second node b is Vb=VA−Vth, so thevoltage sensed with the threshold voltage sensor 400 is VA−Vth where Vthis the threshold voltage imposed on the first TFT T1. Further, thethreshold voltage Vth is obtained after the threshold voltage sensor 400calculates internally. For example, the threshold voltage is thededuction of the voltage VA and the sensed voltage. Afterwards, thethreshold voltage sensor 400 feedbacks the obtained threshold voltageVth, which will be detailed in the following.

The operating principle of the pixel driving circuit in normal displayproposed by the present embodiment of the disclosure is elaborated asfollows. The pixel driving circuit with the 4T1C pixel structureperforms a reset operation (i.e., reset stage), a threshold voltagesensing operation (i.e., threshold voltage sensing stage), a thresholdvoltage compensating operation (i.e., threshold voltage compensatingstage), and a driving operation (i.e., driving emitting stage) in normaldisplay. FIG. 6 is a timing diagram of each of the operating stages ofthe pixel driving circuit in normal display according to the embodimentof the present disclosure. FIG. 7A to FIG. 7D are a set of operatingflowchart of the pixel driving circuit in normal display according tothe embodiment of the present disclosure. The cross symbol (x) on theTFT, as FIG. 7A to FIG. 7D illustrate, means that the TFT stays turnedoff.

At the reset stage, as FIG. 6 and FIG. 7A illustrate, the first scanningsignal Scan1 and the third scanning signal Scan3 are both at highvoltage level; the second scanning signal Scan2 is at low voltage level;the second data signal DATA2 is the sum of the reference signal Vref atlow voltage level and the threshold voltage signal Vth. The source ofthe third TFT T3 and the source of the fourth TFT T4 receive theinitialized signal INI. At this time, the second TFT T2 is turned off;the third TFT T3 and the fourth TFT T4 are both turned on; theinitialized signal INI at constantly low voltage level is written to thefirst node a (i.e., the gate of the first TFT T1) through the third TFTT3; the initialized signal INI at constantly low voltage level iswritten to the second node b (i.e., the source of the first TFT T1)through the fourth TFT T4; the gate and the source of the first TFT Tiare initialized to clear off the remaining data; the first TFT T1 isturned off; the OLED does not emit light.

At the reset stage,Vg=Va=ViniVs=Vb=Vini

Vg indicates the gate voltage level of the first TFT T 1. Va indicatesthe voltage level of the first node a. Vs indicates the source voltagelevel of the first TFT T1. Vb indicates the voltage level of the secondnode b. Vini indicates the constantly low voltage level of theinitialized signal INI.

At the threshold voltage sensing stage, as FIG. 6 and FIG. 7Billustrate, the first scanning signal Scan1 and the third scanningsignal Scan3 are both at low voltage level; the second scanning signalScan2 is at high voltage level; the second data signal DATA2 is the sumof the reference signal Vref at low voltage level and the thresholdvoltage signal Vth; the source of the third TFT T3 and the source of thefourth TFT T4 receive the initialized signal INI. At this time, thesecond TFT T2 is turned on; the third TFT T3 and the fourth TFT T4 areboth turned off; the first node a (i.e., the gate of the first TFT T1)is written to the sum of the reference signal Vref at low voltage leveland the threshold voltage signal Vth (that is, the second data signalDATA2); the voltage level of the second node b (i.e., the source of thefirst TFT T1) is turned into Vref.

At the threshold voltage sensing stage,Vg=Va=Vref+VthVs=Vb=Vref

At the threshold voltage compensating stage, as FIG. 6 and FIG. 7Cillustrates, the first scanning signal Scan1 and the third scanningsignal Scan3 are both at low voltage level; the second scanning signalScan2 is at high voltage level; the second data signal DATA2 is the sumof the display data signal Vdata at high voltage level and the thresholdvoltage signal of Vth. The source of the third TFT T3 and the source ofthe fourth TFT T4 receive the initialized signal INI. At this time, thethird TFT T3 and the fourth TFT T4 are both turned off; the second TFTT2 is turned on; the second data signal DATA2 writes the sum of thedisplay data signal Vdata at high voltage level and the thresholdvoltage signal of Vth to the first node a (i.e., the gate of the firstTFT T1) and the capacitor C through the second TFT 2; the voltage levelof the second node b (i.e., the source of the first TFT T1) is turnedinto Vref+ΔV. ΔV represents the influence of the display data signalVdata at high level on the voltage level of the source of the first TFTT1 (i.e., the second node b). So the influence is irrelevant to thethreshold voltage Vth of the first TFT T1.

At the threshold voltage compensating stage,Vg=Va=Vdata+VthVs=Vb=Vref+ΔV

So the difference Vgs between the gate voltage Vg imposed on the firstTFT T1 and the source voltage Vs imposed on the first TFT T1 isVgs=Vg−Vs=Vdata+Vth−Vref−ΔV.

At the driving emitting stage, as FIG. 6 and FIG. 7D illustrate, thefirst scanning signal Scan1, the second scanning signal Scan2 and thethird scanning signal Scan3 are all at low voltage level; the seconddata signal DATA2 is the sum of the reference signal Vref at low voltagelevel and the threshold voltage signal Vth; the source of the third TFTT3 and the source of the fourth TFT T4 both receive the initializedsignal INI. At this time, the second TFT T2, the third TFT T3, and thefourth TFT T4 are all turned off. The difference Vgs between the firstnode a (i.e., the voltage level of the gate of the first TFT T1) and thesecond node b (i.e., the voltage level of the source of the first TFTT1) maintains the same because of storage of the capacitor C.

Further, the current I flowing the OLED isI=K(Vgs−Vth)² =K(Vdata−Vref−ΔV+Vth−Vth)² =K(Vdata−Vref−ΔV)²,where K indicates an intrinsic conducting factor of the first TFT T1.The intrinsic conducting factor is determined by the characteristics ofthe first TFT T1.

As the equation of the current I flowing the OLED shows, the current Iis irrelevant to the threshold voltage Vth of the first TFT T1. In otherwords, the phenomenon of poor image display due to the drift of thethreshold voltage Vth of the first TFT T1 is completely cleared.

Above are embodiments of the present invention, which does not limit thescope of the present invention. Any modifications, equivalentreplacements or improvements within the spirit and principles of theembodiment described above should be covered by the protected scope ofthe invention.

What is claimed is:
 1. A pixel driving circuit for an organiclight-emitting diode (OLED) display device, comprising: a firstthin-film transistor (TFT), comprising a gate electrically connected toa first node, a source electrically connected to a second node, and adrain electrically connected to a first supply voltage; a second TFT,comprising a gate receiving a second scanning signal and a drainelectrically connected to the first node; a third TFT, comprising a gatereceiving a first scanning signal and a drain electrically connected tothe first node; a fourth TFT, comprising a gate receiving a thirdscanning signal and a drain electrically connected to the second node; acapacitor, electrically connected between the first node and the secondnode; an OLED, comprising an anode electrically connected to the secondnode and a cathode electrically connected to a second supply voltage;wherein when the OLED display device is powered off or powered on, asource of the second TFT receives a first data signal, a source of thethird TFT and a source of the fourth TFT receive an initialized signalor a threshold voltage sensor, the threshold voltage sensor isconfigured to sense a threshold voltage of the first TFT and generate athreshold voltage signal; when the OLED display device operatesnormally, the source of the second TFT receive a second data signalformed by a combination of the threshold voltage signal and a raw datasignal, the source of the third TFT and the source of the fourth TFTreceive the initialized signal; wherein the initialized signal and thefirst data signal are both at constantly low voltage level, and the rawdata signal is at single-pulse high voltage level.
 2. The pixel drivingcircuit of claim 1, wherein the pixel driving circuit performs a resetoperation and a threshold voltage sensing operation when the OLEDdisplay device is powered off or powered on.
 3. The pixel drivingcircuit of claim 2, wherein when the pixel circuit performs the resetoperation, the first scanning signal is at low voltage level, the secondscanning signal and the third scanning signal are both at high voltagelevel, the first data signal is at low voltage level, and the source ofthe third TFT and the source of the fourth TFT receive the initializedsignal.
 4. The pixel driving circuit of claim 3, wherein when the pixelcircuit performs the threshold voltage sensing operation, the firstscanning signal is at low voltage level, the second scanning signal andthe third scanning signal are both at high voltage level, the first datasignal is at low voltage level, and the source of the third TFT and thesource of the fourth receive the threshold voltage sensor.
 5. The pixeldriving circuit of claim 1, wherein the pixel driving circuit performs areset operation, a threshold voltage sensing operation, a thresholdvoltage compensating operation, and a driving operation when the OLEDdisplay device is in normal display.
 6. The pixel driving circuit ofclaim 5, wherein when the pixel circuit performs the reset operation,the first scanning signal and the third scanning signal are at highvoltage level, the second scanning signal is at low voltage level, andthe second data signal is a sum of a reference signal at low voltagelevel and the threshold voltage signal.
 7. The pixel driving circuit ofclaim 6, wherein when the pixel circuit performs the threshold voltagesensing operation, the first scanning signal and the third scanningsignal are at low voltage level, the second scanning signal is at highvoltage level, and the second data signal is a sum of the referencesignal at low voltage level and the threshold voltage signal.
 8. Thepixel driving circuit of claim 7, wherein when the pixel circuitperforms the threshold voltage compensating operation, the firstscanning signal and the third scanning signal are both are low voltagelevel, the second scanning signal is at high voltage level, and thesecond data signal is a sum of the display data signal at high voltagelevel and the threshold voltage signal.
 9. The pixel driving circuit ofclaim 8, wherein when the pixel driving circuit performs the drivingoperation, the first scanning signal and third scanning voltage are atlow voltage level, the second scanning signal at low voltage level, andthe second data signal is the sum of the reference signal at low voltagelevel and the threshold voltage signal.
 10. The organic light-emittingdiode (OLED) display device comprising the pixel driving circuit asclaimed claim
 1. 11. The OLED display device of claim 10, wherein thepixel driving circuit performs a reset operation and a threshold voltagesensing operation when the OLED display device is powered off or poweredon; when the pixel circuit performs the reset operation, the firstscanning signal is at low voltage level, the second scanning signal andthe third scanning signal are both at high voltage level, the first datasignal is at low voltage level, and the source of the third TFT and thesource of the fourth TFT receive the initialized signal; when the pixelcircuit performs the threshold voltage sensing operation, the firstscanning signal is at low voltage level, the second scanning signal andthe third scanning signal are both at high voltage level, the first datasignal is at low voltage level, and the source of the third TFT and thesource of the fourth receive the threshold voltage sensor.
 12. The OLEDdisplay device of claim 10, wherein the pixel driving circuit performs areset operation, a threshold voltage sensing operation, a thresholdvoltage compensating operation, and a driving operation when the OLEDdisplay device is in normal display; when the pixel circuit performs thereset operation, the first scanning signal and the third scanning signalare at high voltage level, the second scanning signal is at low voltagelevel, and the second data signal is a sum of a reference signal at lowvoltage level and the threshold voltage signal; when the pixel circuitperforms the threshold voltage sensing operation, the first scanningsignal and the third scanning signal are at low voltage level, thesecond scanning signal is at high voltage level, and the second datasignal is a sum of the reference signal at low voltage level and thethreshold voltage signal; when the pixel circuit performs the thresholdvoltage compensating operation, the first scanning signal and the thirdscanning signal are both are low voltage level, the second scanningsignal is at high voltage level, and the second data signal is a sum ofthe display data signal at high voltage level and the threshold voltagesignal; when the pixel driving circuit performs the driving operation,the first scanning signal and third scanning voltage are at low voltagelevel, the second scanning signal at low voltage level, and the seconddata signal is the sum of the reference signal at low voltage level andthe threshold voltage signal.